Rocket engines, jet engines, and gas turbines are subject to operational and environmental conditions that can cause material degradation and eventual component failure. Engine health must be monitored during operation, and inspection and preventative maintenance carried out during shut down. It is also desirable to monitor combustion during operation to assure that the combustion process occurs efficiently and as expected.
For rocket engines, such as the Space Shuttle Main Engines (SSME), borescopic inspections are performed after each flight or engine test. The borescope extends an optical probe into closed areas to provide visual inspection. Borescopic inspections are labor intensive and costly. Borescopic inspections are also intrusive, running they risk of introducing contamination into the engine and requiring engine requalification after inspection.
One method to provide real time, non-intrusive monitoring of engine health during operation is spectroscopic monitoring of the engine exhaust plume. Erosion, corrosion, and component failure can deposit engine material in the exhaust plume. Spectrometers can look for anomalous materials in the exhaust plume by monitoring the light spectrum from ultraviolet to infrared. By measuring the frequency and intensity of the light emitted from the exhaust plume, the spectrometer can determine the presence of anomalous materials or unexpected amounts of expected materials by comparison to standard and test measurements. This allows identification of those components which are wearing, corroding, or combusting, and assessment of the degree of wear, corrosion, or combustion. Spectroscopic monitoring in the ultraviolet range also provides an indication of the plume mixture and the combustion process, because the hydroxide radical has a characteristic emission in that range. The magnitude of the characteristic ultraviolet (UV) emission is related to the amount of hydroxide in the exhaust plume.
Optical spectrometers must be able to “see” the exhaust plume to provide an analysis. For hot-test engine firing in a test stand, a remote telescope is often used to feed the image of the exhaust plume to the optical spectrometer. The remote location can produce a poor image if atmospheric conditions are poor, with dust or precipitation in the air. Mounting of the telescopes to focus on a precise portion of the exhaust plume can be difficult. The difficulties are compounded for use of a remote telescope on an engine during flight, since the telescope must track a moving exhaust plume into space.
One approach has been to provide a spectrometer probe near the exhaust plume, attaching it to the engine or to the test stand. The spectrometer probes have been complex, due to optical, heat transfer, and materials problems. Previous designs required lenses and mirrors to direct the optical signal, particularly, high temperature sapphire lenses and a 90-degree reflecting mirror. Dichroic mirrors were required to reduce the infrared reflection on the receiving tip of the fiber optic cable within the probe. Exotic materials present problems in connecting the parts, such as brazing lenses in niobium lens holders, and welding the niobium lens holders to a Inconel® 718 nickel-chromium alloy probe housing to seal the spectrometer probe. Such connection of exotic materials requires development of new processes, which require expensive and difficult certification. The complexity of the spectrometer probe also resulted in a large cross-sectional housing area, about one half inch in diameter, which is exposed to a large heat flux. The complex spectrometer probes have been too expensive, too fragile and too heavy to fly on a vehicle.
Typical spectrometer probes have also focused on the mach diamond, which is the bright area in the engine exhaust plume caused by supersonic phenomena at atmospheric pressure. This presents a tracking problem because the mach diamond moves away from the engine and spreads out with decreasing atmospheric pressure. The mach diamond location also varies with other factors, such as the power level of the engine, the velocity of the emission gases, and the geometry of the nozzle. Tracking increases the chance of error and mechanism failure, and adds weight to the spectrometer probe. An additional tracking problem arises for vehicles with multiple engines: the mach diamonds overlap as they spread with decreasing atmospheric pressure, and each spectrometer probe sees multiple mach diamonds rather than a particular mach diamond.
Lack of a reliable spectrometer probe has prevented integrating exhaust plume monitoring into engine control systems. Although the exhaust plume contains information about engine heath that could be used to tune engine operation and avoid malfunctions by making decisions about adjusting fuel mixture, reducing engine power, isolating engine components, and shutting down the engine, no real time exhaust plume information has been available.
It is desirable, therefore, to provide an engine spectrometer probe and method of using the same that overcomes the above disadvantages.